Solid-state image sensors include a plurality of photodetectors, such as photodiodes, arranged in either a line array or a rectangular array of rows and columns. A shift register extends along the line or each column of photodetectors to carry the collected charge carriers to an output circuit. Charge-coupled devices (CCD's) are a preferred form of shift register used in these image sensors. CCD's, in general, include a channel region along which the charge carriers travel and gates over the channel region for moving the charge carriers along the channel.
One type of CCD is a virtual phase CCD. This type of CCD can include a plurality of gates spaced along the channel and electrically connected together to operate in unison. Between each pair of adjacent gates and within the channel region are regions of different doping levels to form potential wells of various depths increasing from one gate to the next along the path of the channel region. Along the surface of the channel region between the gates is a virtual gate which fixes the potential level of the portions of the channel region between the gates. The virtual gate is a high conductivity region along the surface of the channel region.
One such virtual gate CCD is shown in U.S. Pat. No. 4,229,752 to J. Hynecek, issued Oct. 21, 1980, entitled "Virtual Phase Charge Transfer Device". This patent also describes several methods of making such a virtual phase CCD. One of the methods described in the Hynecek patent requires a diffusion from an oxide layer. However, this is unsatisfactory since the inward diffusion is difficult to control and some of the dopant remains in the oxide layer where it is not needed. A more serious drawback of this method is that the dopant tends to excessively diffuse at the very edge of an electrode of the CCD both downwardly and laterally outwardly, which caused unwanted alterations of the potential well of adjacent regions being formed. A second method described in the Hynecek patent has the drawback that it can result in a device which is susceptible to transfer inefficiency and image smear as a result of particulate masking of some of the ion implantations used to form the CCD. If particles fall on the substrate just before the second acceptor implantation, potential wells can be formed in the gated storage region which may trap charge. Furthermore, this second method requires a masking layer to be etched twice which can adversely effect the underlying layers unless very stringent selective etching is used. Thus, the methods of the Hynecek patent require a number of steps which must be tightly controlled or undesirable modifications to the CCD being formed can result.